The present invention relates to a printing apparatus and method for a bonding material, in which a board is supported from its component mounting-side surface on which a plurality of components are mounted while a bonding material of components is printed on its working surface that is a surface opposed to the component mounting-side surface of the board.
Higher integration is required from electronic circuits, which are formed by mounting a plurality of components or electronic components on a board, as electronic equipment incorporating such electronic circuits has higher performance and smaller size. Accordingly, in recent years, in order to make efficient use of the surface area of the board, the electronic components are mounted on both sides of the board, i.e., double-sided mounting is performed (see, e.g., Japanese unexamined patent publication No. H04-97587 or No. 2003-258419).
Moreover, such mounting of the electronic components is generally achieved by a hot-air reflow method which is composed of the steps of: printing a solder paste, which is a bonding material, at component mounting positions on a board (printed circuit board); then disposing electronic components on the component mounting positions; and melting the solder paste by such means as blowing hot air to the entire board so as to conduct soldering.
Moreover, in the case where the double-sided mounting is performed, one side of the board has respective electronic components mounted thereon through the steps of: printing a solder paste on the side of the board by means of screen printing and the like; disposing respective electronic components through the solder paste; and reflowing, after which the other side has respective electronic components mounted thereon through the same procedures.
A description is herein given of the procedures of a conventional method for feeding a solder paste to the other side when such double-sided mounting is performed, i.e., the procedures of a conventional screen printing method, with reference to schematic explanatory views in FIGS. 22A, 22B and 22C.
First, FIG. 22A shows a board 502 with electronic components 501 mounted on its one surface in the state of being transported to a printing position while the one surface faces down and opposite end portions on the one surface are supported by belt conveyer transportation rails 503a and 503b. 
After that, as shown in FIG. 22B, at the printing position A, a support block 504 for supporting the lower surface of the board 502 so as to support the board 502 is on standby, while above the support block 504, a screen 506 whose periphery is supported by a screen frame 505 is also on standby. In this state, at the printing position A, the board 502 supported by the respective transportation rails 503a and 503b are transported in between the screen 506 and the support block 504.
Upon this transportation, as shown in FIG. 22C, the support block 504 is elevated so that the support block 504 supports the lower surface of the board 502 and the supported board 502 is lifted. The lifted board 502 is brought into the state that its upper surface is in contact with a plate surface portion of the screen 506, after which elevation of the support block 504 is stopped and the contact state is retained. It is to be noted that although the lower surface of the board 502 has a plurality of the electronic components 501 mounted thereon, a recess portion 550 is formed on the upper surface of the support block 504 depending on the mounting position and shape of the respective electronic components 501, thereby allowing reliable support of the board 502 by the support block 504.
After that, as show in FIG. 22C, the upper surface of the screen 506 which is in contact with the upper surface of the board 502 is scrabbled by a squeegee unit 507 so that a solder paste 508 is fed to the board 502 through the screen 506, by which printing of the solder paste is conducted.
Herein, a schematic plane view of the one surface (i.e., the lower surface) of the board 502 is shown in FIG. 23. As shown in FIG. 23, the board 502 is in an almost square shape, and along the inner outline of the almost square shape, slit-like-hole portions 509 are formed. The slit-like-hole portions 509 are formed so that after completion of mounting of the electronic components 501, a plurality of slim connecting portions 512 which connect an internal component portion 510 and an outline portion 511 (generally referred to as an edge portion) are formed to make it easy for the product portion 510 (electronic circuit) to be cut off and separated from the outline portion 511. It is to be noted that the region surrounded with the respective slit-like-hole portions 509 is a component mounting region (product portion 510), on which a plurality of the electronic components 501 are mounted.
Moreover, as shown in FIG. 23, respective edge portion regions 513a, 513b in the vertical direction in the drawing are the regions supported by the belt conveyer transportation rails 503a and 503b shown in FIG. 22A. The support block 504 can support the board 502 in the regions other than these edge portion regions 513a, 513b, and a width dimension W1 of the support block 504 is a dimension obtained by subtracting width dimensions of the respective edge portion regions 513a, 513b from a width dimension W2 of the board 502.
In recent years, high-density mounting of the electronic components 501 on the board 502 is more and more progressing, along with which the number of the electronic components 501 mounted on the board 502 increases and the shape and size of the respective electronic components 501 to be mounted becomes of great variety. Further, the support block 504 having the above-described structure is required to have the recess portion 550 depending on the shape and size of the respective electronic components 501, which increases the forming places of support faces 551 for supporting respective non-mounting regions 514a, 514b, 514c in the board 502. However, in the case where operations such as the screen printing are reliably performed, while it is necessary to maintain the board 502 to a near-horizontal position, forming the recess portion 550 and a plurality of the support faces 551 having the size and shape in such variety on the support block 504 makes it more difficult to maintain the board 502 to a near-horizontal position in terms of its processing precision and the like.
Further, there is a support block in which, for reliable retention of the board 502, suction holes are formed on the respective (or any) support faces 551 of the support block 504 and the supported board 502 is sucked through the suction holes so that the board 502 is sucked and held by the support faces 551. Further, in response to demands for high-density mounting which exhibits more and more progress in recent years, the number of the respective electronic components 501 to be mounted on the board 502 or its mounting density are increasing, so that the area of the non-mounting regions 514a, 514b and 514c, which are the regions for supporting the board 502 in the state of being in contact with the board 502, is on the considerable decrease. Under these circumstances, it becomes difficult to form the suction holes on the respective support faces 551, which in turn makes it difficult to reliably suck and retain the supported board 502.
Moreover, in order to respond to retention of the board 502 having a variety of electronic components 501 mounted thereon, there has been devised a method for supporting the board 502 with the support block, in which a soft layer is formed on the upper face (support face) of the support block, and the electronic components 501 in contact with the soft layer are supported by deforming the soft layer. However, such a method is effective when all the electronic components 501 to be mounted are relatively small in thickness to the extent that the soft layer can be sufficiently deformed. Therefore, in the case where the respective electronic components 501 are varied in thickness, i.e., in the case where some electronic components 501 are larger in thickness than other electronic components 501, the soft layer cannot be sufficiently deformed, thereby causing a problem in that reliable support of the board 502 cannot be achieved.
Further, in the board 502 whose one side already has components mounted thereon, thermal hysteresis caused by the loading of the components often generate warpage on the board 502 as shown in a virtual line in FIG. 22A, and further in the case where a fewer number of the suction points are left on the product portion 510, there is a problem in that warpage straightening and retaining effects on the board 502, which are obtained by sucking and holding the board 502 so as to bring the board 502 in close contact with the support faces 551 on the support block 504, cannot be expected.
Moreover, even in the case where the warpage straightening and retention effects by the support block 504 can be obtained, if an adhesion force between the screen 506 and the upper surface of the board 502 is too large to achieve sufficient screen release when the support block 504 is lowered and the board 502 is separated (released) from the screen 506 after printing, the screen 506 is gradually separated from the board 502 while partially being stretched downward as shown in FIG. 24. In such a case, at the time when a portion which sticks to the screen 506 till the last is separated from the board 502, the screen 506 vibrates in the vertical direction due to the action of the separation, which may cause printing results to have bleeding and dropout failures, thereby hindering high-precision printing.